The present invention relates in general to integrated circuits and, more particularly, to reverse voltage protection of an integrated circuit.
Most if not all integrated circuits require a power supply potential to energize the internal transistors for proper operation. In many applications, it is possible to inadvertently reverse the polarity of the power supply potential. For example, in an automotive application, the battery may be connected backwards with the negative supply to the positive power supply conductor. Without some form of reverse voltage protection, such a reverse voltage connection could damage integrated circuits (IC) within the automobile.
One known technique is to place a high current discrete diode in series between the battery and the positive power supply bus that goes to the ICs requiring protection. A reverse voltage from the battery simply reverse biases the diode and protects the ICs. Although a fully charged battery typically supplies 12.0 volts DC, automotive manufacturers are demanding the ICs continue to operate should the battery voltage drop as low as 5.0 volts DC. The discrete diode typically has a forward voltage drop of 1.0 volt and increases with more current flow. In high current applications, the large forward voltage diode drop leaves only 4.0 volts or less operating potential headroom for the ICs. In some IC designs, 4.0 volts DC may not provide sufficient operating potential headroom for MOS devices over a full temperature range.
Another option is to place a conventional lateral PNP transistor configured as a diode in series with the positive power supply conductor on each IC that requires reverse voltage protection. Placing one diode-configured transistor on each IC keeps the forward voltage drop to acceptable limits. Unfortunately, the conventional lateral PNP diode requires many emitters to handle the current load of say up to 100 milliamps (Ma) because each emitter can typically handle only 0.5 Ma. The large number of emitters (200+) cause the lateral PNP diode to consume an enormous area on the IC die.
Hence, a need exists for reverse voltage protection that can handle a wide range of supply current and that does not consume excessive IC die area.